The last time half of North America was under ice was 20kya, 8kya was relatively warm with just a bit of ice that hadn't melted yet in the northeast of Canada.

The last time half of North America was under ice was 20kya, 8kya was relatively warm with just a bit of ice that hadn't melted yet in the northeast of Canada. The species that entered those areas would have done so during climate-driven redistributions from the south as entire plant and animal communities moved north to fill new available habitat, and while there would have been some local adaptation, it isn't like they all evolved from vastly different ancestors in 20k years. > But other animals and plants are currently filling the niche and containing to expand. Sort of. For example, the main thing that has filled the niche of predator of grazers like deer across eastern North America is probably the automobile. In terms of biomass, yes there's a lot of domestic animals. But in terms of functional diversity, there has been a great decline. Just think of the difference between prarie and a crop monoculture. The fewer species in each guild that interact to form the ecosystem, the less resilience overall. Despite large amounts of biomass, the loss of biodiversity is undeniably an issue.